Control circuits for electromagnetic chucks



July 20, 1948. D, SNYDER 2,445,459

CONTROL CIRCUITS FOR ELECTROMAGNETIC CHUCKS Filed Sept. 7, 1944 I I l fJTIOff I75 9 1o 38 32 f? WITNESSES: 34 s INVEN"I'OR l y 52% 37 40 BY wATTORNEY rent continues to flow.

Patented July 20, 1948 2,445,459 CONTROL CIRCUITS FOR ELECTRO- MAGNETICCHUCKS Frederick D. Snyder,Milton, Westinghouse Electric Corporation,East Pitts- Mass., assignor to burgh, Pa., a corporation of PennsylvaniaApplication September. 7, 1944, Serial No. 553,034 4 Claims. (01.175-335) My invention relates paratus which embody to electromagneticapmagnetizing and demagnetizing the holding magnets of chucks orretentive relays and the like magnetic devices.

Electromagnetic chucks, as used on machine a holding magnet, and moreparticularly to electric control circuits for tech a magnetizableworkpiece to the ole surface of the chuck as long as the energizing cur-Upon interruption of this as used on cranes.

It is an object of my invention to provide concurrent excitation. Adirect-current of reverse polarity or an alternating current of lowamplitude is usually applied in order to release the contactor byreducing the attractive force of its residual magnetism below thatrequired for overcoming the opening tendency of the armature bias.Referring to such contactors, it is also an object of my invention ingor delay of the release performance.

In order to achieve these objects, and in ac- :ordance with myinvention, I provide an electric holding magnet of the type referred towith a capacitor which is charged previous to the release operation andwhich is caused to pass a demagnetizing discharge through the magnetcoils for canceling or reducing the residual magnetism of the holdingmagnet. The capacitor and coil circuit is so rated as to produce ademagnetizing effect suflicient to eliminate the residual flux in one ora few successive discharge operations.

According to another feature of the invention the control circuit of aholding magnet is equipped with a control switch whose operation, whenreleasing the magnet, causes a capacitor to be charged and dischargedtwice or several times so that the integral eflect of the severaldischarges results in the desired demagnetization. According to stillanother feature, related to the one just mentioned, each subsequentcharge imposed on the capacitor is caused to flow through themagnetizing coils of the holding magnet in the direction opposite to theoriginal magnetizing current so as to contribute also to reducing theresidual induction.

In still another aspect of my invention, I provide charging means forthe above-mentioned capacitor which include a potentiometric circuit soas to permit a desired adjustment of the char ing voltage of thecapacitor or a change of this voltage in accordance with the particularrequirements of each case of application.

In conjunction with any of the features mentioned in the foregoing andin accordance with a further essential point of the invention, thecapacitor for issuing a demagnetizing current is so rated relative tothe coils of the magnet that it forms an oscillatory system therewithwith the efiect of producing an alternating discharge current of asubstantially given frequency and attenuation.

Lastly, it is also a feature of the invention to apply a capacitordischarge, in the manner referredto in the'foregoing, to anelectromagnetic contactor of the retentive type so that demagnetizationof the relay involves a rated or metered timing effect which imparts tothe relay the characteristic of a timing device.

4 These objects and features of the invention will be apparent from thefollowing description of the embodiments illustrated in the drawing inwhich:

Figure 1 shows a top view of part of a holding magnet appertaining .toan electromagnetic chuck, the coils of the magnet being omitted in inthis illustration;

Fig. 2 is a diagrammatic showing of a section .REISSUED APR 4 1950through the holding magnet shown in Fig. 1

and indicates also the appertaining coils;

Fig. 3 is a control circuit according to the invention for operating aholding magnet as shown in Figs. 1 and 2;

Fig. 4 shows diagrammatically another embodiment of a control circuitfor chucks;

Fig. 5 is a similar circuit diagram representing a third embodiment ofthe invention;

Figs. 6 and '7 are explanatory and show different current-time curvestypical of the demagnetizing discharge current occurring in capacitivecircuits according to the invention; and

Fig. 8 is the circuit diagram of an electroma netic relay having aretentive magnetic circuit provided with control means according to theprinciples of the invention.

Referring to Figs. 1 and 2, the illustrated holding magnet is chosen asrepresentative of a large variety or available chucks. The chuck, asshown, is intended for a machine tool in which the body I of the magnetis mounted on a reciprocating machine portion for carrying a workpiecealong a stationary tool. However, a similar chuck may also be used as astationary machine part for holding a workpiece relative to a movabletool; and it will also be understood that the principles of my inventionare likewise applicable to chucks of the rotating type such as used onlathes. The

body I of the holding magnet is provided with spaced projections 2forming between them a series of grooves 3. The grooves 3 are providedwith coils denoted by 4 and 5. These coils are all connected with oneanother and so wound that the projections 2 form alternating north andsouth poles as is indicated in Fig. 2 by the letters "N and "B,"respectively. The top surfaces of the projections 2 form the supportingsurface for the workpiece B of magnetizable material. When the chuckwindings are energized, the magnetic flux issuing from each north poleto the adjacent south poles passes through the workpiece B and producessufiicient attraction to hold the workpiece firmly against the chuck.This holding force obtains as long as the energizing direct currentpasses through the coils. Upon interruption of the current, theattractive force is reduced. However, the remanent magnetic induction ofthe holding magnet i, or of the workpiece B, or both, remains, as arule. sufiicient to prevent lifting the workpiece from the chuck. Hence,it is necessary to apply a demagnetizing force to the magnet. This isalso desired in cases where the persistence of an appreciable remanentmagnetism in the workpiece B after its removal from the chuck isundesirable.

The control circuit of coils l and 5 is schematically shown in Fig. 3.In this figure, the totality of coils is represented schematically by asingle winding W. This winding is connected to directcurrent terminals 6and I through a single-pole on-and-oi'l contact. A capacitor C isconnected across the winding W. This capacitor C is so rated relative tothe winding W that a charge impressed on the capacitor by the linevoltage suffices to store sufficient energy for demagnetizing thewinding. When switch 8 is closed the winding W is energized and inducesin the chuck the magnetism necessary for firmly attaching the workpiece.Switch 8 remains closed as ong as the chuck is in operation. During theclosure of switch 8, the capacitor ,C collects a charge. When the switch8 is opened thereby interruptin the magnetizing current, the capacitordischarges itself through winding W. The discharge may occur asrepresented by the voltage-time curve D in Fig. 6. As long as switch 0is closed the voltage across the capacitor C is at its maximum value E.At the moment of interruption, denoted by Ti in Fig. 6, the capacitorbegins issuing its charge through winding W. The winding and capacitorform an oscillatory system so that the discharge current is alternating.Due to the fact that the magnitude of this current declines gradually,the alternating magnetizing force imposed on the magnetic circuit hasthe effect of reducing the magnetic remanence. Hence, upon cessation ofthe discharge, the workpiece can be lifted from the chuck.

A control circuit as described in the foregoing is suiiicient forsmaller chucks. In order to obtain a suflicient demagnetization also incase of larger chucks and workpieces without requiring capacitors ofextremely large rating, a repeated demagnetizing performance ispreferably provided for. To this end, the control circuit may bedesigned as represented in Fig. 4. According to Fig. 4, the windin W andthe capacitor C are connected to direct-current supply leads 9 and I0through a multi-position control switch S. In the embodiment of Fig. 4,this control switch S is of the rotary type and has four positions. Inorder to facilitate explaining a full cycle 01 operation of the switch,its contact elements are illustrated in developed form, and one of itsfour positions, i. e. the oiP' position, is shown twice. The controlswitch 5 has a body ll of insulating material which is rotated by meansor a shaft i2 and an operator-actuable handle II. A number of stationarycontact fingers, such as those denoted by H, co-operate with contactsegments I! through is which are mounted on the rotatable body H. Thedirection of motion is indicated by the arrow marked A. A rheostat isdenoted by R.

In the illustrated ofi" position of the switch, the winding W isshort-circuited by the capacitor C and disconnected from the leads 9 andii of a direct current line. The chuck is nowin condition to receive aworkpiece. Upon placing the workpiece on the chuck, switch S is turnedto position I. In this position, lead 9 is connected through segment ISwith one terminal of winding W, while the other terminal of the windingis connected through segment H with lead ll. Consequently the chuckcoils are now energized so that the workpiece is firmly attached to thechuck. The capacitor C remains connected across winding W by segment l1and hence is charged. In order to release the workpiece, the switch S isturned into position II. This has the eflect of disconnecting winding Wfrom both leads 9 and I0, while the capacitor C remains connected acrossthe winding. As a result, the capacitor passes a demagnetizing dischargecurrent through the winding in the manner described above. Uponcontinuance of its rotation, switch S passes through its position III.In this position, the left-hand terminal of winding W remainsdisconnected so that the winding stays de-energized. The capacitor C,however, is now connected at one of its terminals to lead l0 throughsegment It. The other terminal of the capacitor is connected throughsegment i8 to the slider of rheostat R. At the same time, the resistoror rheostat R is connected through segment I! across leads 9 and i0.Consequently, the capacitor C is now charged from the line under avoltage determined by the setting of the rheostat, and this voltage isof opposite polarity as compared with that previously eflective acrossthe capacitor, and the voltage is so chosen by correspondinglypositioning the rheostat slider that the subsequent discharge sumces todemagnetize the-chuck to the extent needed for permitting the removal ofthe workpiece.

In the last position of switch 8, which is identical with the "01?position, the capacitor C is again connected across winding W and bothare disconnected from the line.

The embodiment shown in Fig. involves a further improvement over that ofFig. 4. The control circuit of winding W and capacitor C includes arheostat R and a control switch S, as in the embodiment of Fig. 4. Theswitch S has its rotor 2i provided with seven segments denoted bynumerals 25 through 3| for co-operation with stationary contact fingerssuch as those denoted by 24. The switch has four contact positions ofwhich the "ofl" position is shown twice in the developed diagram. In the"01? position, winding W and capacitor C are connected in parallelthrough segments 21 and 30 and are both disconnected from the line. Inposition I, lead I is connected through segment 25 with one terminal ofwinding W, while the other terminal of the winding is connected throughsegment 2! with lead 3. The capacitor remains in parallel to winding W.Hence the chuck is now magnetized and the capacitor charged. In positionII of the switch, winding W is disconnected from the line so that thecapacitor passes a discharge current through the winding as in theprevious embodiments. In the third position, lead 9 is connected throughsegment 26 to one end of winding W, while the other end of the windingis connected through segment 29 with one pole of the capacitor C. Theother pole of the capacitor is connected through segment 28 with theslider of rheostat R. The rheostat R is connected across the linethrough segment 3|. Consequently, in this third'position, the capacitoris charged in accordance with the voltage setting of the rheostat R, butin contrast to the embodiment of Fig. 4, the recharging current passesalso through the winding W. This current, however, is limited to thedisplacement current of the capacitor and its direction of flow inwinding W is opposite to that of the original demagnetizing current. Asa result, the recharging current of capacitor C performs also ademagnetizing efiect. In the last position of switch S which isidentical with the initial ofi position, the capacitor is againdischarged through winding W. It will be noted, however, that thissecond discharge has a polarity opposite to the first discharge astypified by the voltage time curve F in Fig. '7. The discharge begins atthe moment T2, and its first and largest magnitude is in thedemagnetizing direction of the holding magnet.

It will be apparent from the above described examples that the use of acapacitive discharge current for producing a demagnetizing effectaccording to the invention involves the application of a metered orlimited demagnetizing effect which secures a definite reduction inremanent magnetism each time it is applied to the magnet coils. Hence,the control means according to the invention eliminate theabove-mentioned trial and error method, and permit obtaining a sumcientdemagnetization without requiring any skill or timing in the actuationof the control switch, repeating the capacitive charging and dischargingoperation if necessary. Chucks for machine tools designed in accordancewith Figs.

4 and 5 are usually suflicient to produce substantially totaldemagnetization by a single full-cycle operation of the control switch.01' course, if desired, the recharging and discharging stepscorresponding to switch position III and 03" may be repeated.

The embodiment of a magnetically retentive contactor shown in Fig. 8serves to exemplify the use of the invention for obtaining a timingeiifect. According to Fig. 8, the magnetic circuit of a relay formed bya stationary fleldpiece 32 and a movable armature 33 containsmagneticallyretentive material so that the armature 33 remains attractedand sealed against the fieldpiece 32 once the latter has been magnetizedsufliciently. The fieldpiece 32 is provided with a magnetizing coil 34and a demagnetizing coil 35. Coil 33 is connected through an interlockcontact 36 and a control contact 31 between the direct-current terminals33 and 39 of the relay circuit. A contact 33 permits connecting acapacitor 0 across the direct-current terminals in series with a currentlimiting resistor 4|.

The armature 33 is provided with a main contact assembly (notillustrated) to be controlled by the relay. The interlock contact 33,which may form part of the main assembly, is closed when the armature isin the open position, and is interrupted when the armature is attractedby the fieldpiece and has moved through most of its travel toward thefieldpiece. The two contacts 31 and 43 are interconnected so that thecontact 30 shifts from one to its other stationary connections whencontact 31 is closed and opened.

In the condition shown in Fig. 8, the magnetic circuit of the relay isde-energized. When contact 31 is closed, coil 33 is energized so thatsumcient magnetism is induced in fieldpiece 32 for attracting thearmature 33. The armature moves upwardly and, near the end of itstravel, interrupts the magnetizing circuit at contact 33. Although theenergizing current is now interrupted, the armature 33 remains inpicked-up position due to the residual magnetism of the magneticcircuit. At the same time, contact 43 connects capacitor C acrossterminals 33 and 33 so that the capacitor is charged through resistor4|. In order to release the relay, contact 31 is opened. This causescontact 40 to connect the capacitor C across coil 35. As a result adischarge current passes through the coil and demagnetizes the magneticcircuit until the residual magnetism becomes lower than is necessary forovercoming the armature bias. The armature will then drop oiI so thatthe relay system is in its original condition and ready for a newoperation.

The demagnetizing current flowing in the oscillatory system of coil 35and capacitor C reduces the retentive induction of the magnetic circuitduring each second half cycle of an alternating magnetizing forceoccurring substantially in accordance with a curve of the type shown inFig. 6 or 7. The effect of this stepwise demagnetization iscumulativeand hence requires the occurrence of a multitude of alternating cyclesbefore the relay drops oil. In other words, the demagnetizing efiect isnot instantaneous but extends over a period of time which depends on therating of the capacitive charge and the frequency and attenuation of thedischarge current. A change in any of these determinants permitschanging and adjusting the timing period of the relay.

As exemplified by the various embodiments described in the foregoing andas will be understood by those skilled in the art upon a study oi thisdisclosure, control systems according to' the principles oi my inventionmay be modified and altered as to details without departing from thegist and scope of the invention as set forth in the claims attachedhereto.

I claim as my invention:

1. With a magnetic chuck having a holding magnet, the combination ofcoil means inductively associated with said magnet, circuit means forenergizing said coil means from a directcurrent source, a capacitor, aselective control switch movable between at least three sequentialpositions and having three sets of contacts operative when said switchis in said respective three positions, a first one of said sets ofcontacts being connected to said coil means and said capacitor and saidcircuit means so as to magnetize said magnet and charge said capacitorwhen said switch is in one of said positions to substantiallydemagnetize the chuck when said switch is in the next position, a secondone of said sets of contacts connecting said coil means across saidcapacitor for discharging said capacitor through said coil means, andthe third set 01' contacts being connected to said circuit means andsaid capacitor for recharging said capacitor while maintaining said coilmeans disconnected from said capacitor and said circuit means when saidswitch is in the third position.

2. With a magnetic chuck having a holding magnet, the combination ofcoil means inductively associated with said magnet, circuit means forsupplying energizing direct current to said coil means, a capacitor, acontrol switch disposed between said coil means, capacitor and circuitmeans and having at least four sequential positions and four sets ofrespective contacts of which only one set is effective at a timedepending upon the position of said switch, one of said sets of contactsbeing connected to said circuit means and said coil means and saidcapacitor to magnetize said magnet and charge said capacitor when saidswitch is in the first position, each of the second and fourth sets ofcontacts being attached only to said coil means and said capacitor andconnecting said capacitor across said coil means for discharging saidcapacitor through said coil means to substantially demagnetize the chuckwhen said switch is in the second and fourth positions respectively, andthe third set of contacts connecting said capacitor only to said circuitmeans for recharging said capacitor when said switch is in the thirdposition.

3. With a holding magnet, the combination of coil means inductivelyassociated with said magnet, circuit means for supplying energizingdirect current to said coil means, a capacitor for substantiallydemagnetizing said magnet by discharging through said coil means, acontrol switch having at least three positions and three respective setsof contacts of which only one set is eflective at a time depending uponthe position oi said switch, a first one of said sets of contactsconnecting said circuit means to said coil means and said capacitor tomagnetize said magnet and charge said capacitor by voltage of onepolarity when said switch is in one position, a second one of said setsof contacts being connected only to said coil means and said capacitorfor discharging said capacitor through said coil means when said switchis in another position, and the third set of contacts connecting saidcircuit means only to said capacitor and having reversed polarity oiconnection as compared with said first set of contacts so as to chargesaid capacitor by voltage of opposite polarity when said switch is inthe third position.

4. With a magnetic chuck having a holding magnet, the combination ofcoil means inductively associated with said magnet, circuit means forenergizing said coil means from a directcurrent source, a capacitor, apotentiometric rheostat, and control means having a selector switch withat least three selective positions, said switch having three sets ofcontacts of which only one set is eflective at a time depending upon theposition of said switch, one of said sets of contacts connecting saidcircuit means to said coil means and said capacitor to magnetize saidmagnet and charge said capacitor when said switch is in one of saidpositions, another one 0! said sets of contacts connecting said coilmeans across said capacitor for discharging said capacitor through saidcoil means when said switch is in another position to substantiallydemagnetize said magnet, and the third set of contacts conmeeting saidcircuit means to said rheostat and said capacitor so as to recharge saidcapacitor through said rheostat when said switch is in the thirdposition.

FREDERICK D. SNYDER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

